Department of Epidemiology and Public Health, Yale University School of Medicine, 606 LEPH, New Haven CT 06510, USA.

Abstract

Symbiotic associations often enhance hosts' physiological capabilities, allowing them to expand into restricted terrains, thus leading to biological diversification. Stable maintenance of partners is essential for the overall biological system to succeed. The viviparous tsetse fly (Diptera: Glossinidae) offers an exceptional system to examine factors that influence the maintenance of multiple symbiotic organisms within a single eukaryotic host. This insect harbours three different symbionts representing diverse associations, coevolutionary histories and transmission modes. The enterics, obligate mutualist Wigglesworthia and beneficial Sodalis, are maternally transmitted to the intrauterine larvae, while parasitic Wolbachia infects the developing oocyte. In this study, the population dynamics of these three symbionts were examined through host development and during potentially disruptive events, including host immune challenge, the presence of third parties (such as African trypanosomes) and environmental perturbations (such as fluctuating humidity levels). While mutualistic partners exhibited well-regulated density profiles over different host developmental stages, parasitic Wolbachia infections varied in individual hosts. Host immune status and the presence of trypanosome infections did not impact the steady-state density levels observed for mutualistic microbes in either sex, while these factors resulted in an increase in Wolbachia density in males. Interestingly, perturbation of the maternal environment resulted in the deposition of progeny harbouring greater overall symbiont loads. The regulation of symbiont density, arising from coadaptive processes, may be an important mechanism driving inter-specific relations to ensure their competitive survival and to promote specialization of beneficial associations.

Mutualistic symbiont density through host development and mating status. (a) Sodalis density. Sample sizes are indicated adjacent to each time point. (b) Wigglesworthia density, Wigglesworthia was not detectable (n.d.) in the majority of L1 samples. Male and virgin female densities are represented with solid and hatched lines, respectively (n≥5 samples at each time point for each sex). Each time point refers to a mean, error bars represent 1 standard error of the mean (s.e.m.). *, ** indicate significant differences between corresponding time points (p<0.05, 0.005; respectively). (c) Sodalis and Wigglesworthia densities in mated females. Sodalis (black bars) and Wigglesworthia (white bars) densities in 2, 4, 6 and 8 week old mated females. Each time point refers to a mean, error bars represent one 1 s.e.m. (n≥5 samples at each time point).

Wolbachia density through host development and in different offspring. (a) Box-and-whiskers plots of Wolbachia density and distribution of variance through host development. Host development was separated into three discrete categories; larvae (n=30), pupae (n=109) and adult (n=58). The median (heavy line) ranged from the 25th to 75th percentile (boxed) and 10th and 90th percentiles (whiskers) are shown. (b) Wolbachia densities from 1st, 2nd, 3rd and 4th offspring of individual mothers (A–J) measured immediately following larval deposition.